Liquid crystal devices such as LCD's have poor heat resistance. When they are bonded to FPC's (flexible printed circuits), an anisotropic conductive material such as an anisotropic electric-conductive adhesive is used. The anisotropic conductive material achieves thermocompression bonding and which comprises a binder component in the form of a thermosetting resin such as an epoxy resin and electric-conductive particles such as Ni particles. This anisotropic conductive material, which comprises a thermosetting insulating resin in which spherical electrically conductive particles are dispersed, is sandwiched between opposing substrates. By applying pressure in the thickness direction while heating, anisotropy is achieved in which there is electrical conductivity in the vertical direction and insulating properties between adjoining conductors in the horizontal direction.
The state in which substrates are bonded to each other using an anisotropic conductive material will be explained. FIG. 1 is a schematic view for explaining the state before bonding of substrates to each other, and FIG. 2 is an explanatory view for explaining the state after bonding.
As shown in FIG. 1, a plurality of conductors 2 are formed on a lower substrate 1, and a plurality of conductors 4 are formed on an upper substrate 3, which is bonded to the lower substrate, in the same location as the conductors 2 of the lower substrate. The lower substrate 1 and the upper substrate 3 are positioned so that the corresponding conductors 2 and 4 are aligned, and an anisotropic electrically conductive film 5 which is an anisotropic conductive material is sandwiched between the lower substrate 1 and the upper substrate 3. A large number of electric-conductive particles 7 are dispersed in a thermosetting resin 6 of the anisotropic electrically conductive film 5.
As shown in FIG. 2, when a heater block 8 is disposed atop the upper substrate 2 and presses downwards while heating, thermocompression bonding takes place, and the anisotropic conductive film 5 softens and is sandwiched between the upper and lower conductors 2 and 4. At this time, the electric-conductive particles 7 which are sandwiched between the upper and lower conductors contact the upper and lower conductors 2 and 4 and can conduct between the conductors. If heating further proceeds, the thermosetting resin 6 liquefies and adheres the lower substrate 1 and the upper substrate 3 to each other. The electric-conductive particles between the adjoining conductors are surrounded by the thermosetting resin and do not contact each other, and they are present in the thermosetting resin without conducting. If heating by the heater block 8 is continued and the curing temperature of the thermosetting resin is reached, the thermosetting resin 6 is cured and the upper and lower substrates are securely bonded to each other. This is the bonding mechanism of substrates using an anisotropic conductive material.
Electric-conductive particles used in anisotropic conductive materials include high melting point particles and low melting point particles. High melting point particles include metallic particles of gold, silver, nickel, and the like, as well as non-metallic particles of ceramics or plastics having a surface coating of nickel, gold, or the like. These high melting point particles do not melt at the time of thermocompression bonding and maintain a spherical shape after bonding. Low melting point particles include Pb-63Sn, Pb-5Sn, Sn-3.5Ag, Sn—In, Sn—Cu, Sn—Ag, Sn—Zn, Sn—Zn—Bi, Sn—Ag—Bi, Sn—Ag—Bi—In, and the like. Low melting point particles melt at the time of thermocompression bonding. (Patent Documents 1-7)
In an anisotropic conductive material, the electric-conductive particles conduct between upper and lower conductors, but there is a great difference between high melting point particles and low melting point particles with respect to the state of the electric-conductive particles when they are conducting. Here, the state in which high melting point particles or low melting point particles are conducting between conductors will be explained.
FIG. 3 shows the state of conduction between upper and lower electrodes with high melting point particles. When high melting point particles 7(K) which are positioned in the space 9 between the conductors 2 of the lower substrate 1 and the conductors 4 of the upper substrate 3 undergo thermocompression bonding in a thermocompression bonding apparatus, first, when the thermosetting resin 6 softens, the high melting point particles 7(K) push the thermosetting resin 6 out of the way and reach the conductors 2 and 4, and the high melting point particles 7(K) contact and conduct between the conductors 2 and 4. Since this contact is contact between the planar conductors and the spherical high melting point particles, it is point contact in which a portion of a sphere contacts a plane. When the temperature is further increased and the thermosetting resin liquefies, the liquefied thermosetting resin adheres to the upper and lower substrates 1 and 3. As heating progresses and the curing temperature of the thermosetting resin is reached, the thermosetting resin cures and firmly bonds the upper and lower substrates 2 and 4 to each other. At this time, the high melting point particles 7(K) in the spaces 10 between adjoining conductors are not contacting the conductors, so they do not affect conduction.
FIG. 4 shows the state of conduction between upper and lower conductors when using low melting point particles. When the low melting point particles 7(T) disposed in the space 9 between a conductor 2 of the lower substrate 1 and a conductor 4 of the upper substrate 3 are subjected to thermocompression bonding in a thermocompression bonding apparatus, first, when the thermosetting resin 6 softens, the low melting point particles 7(T) push the thermosetting resin 6 out of the way and reach the upper and lower conductors 2 and 4, and the low melting point particles 7(T) contact the conductors 2 and 4. If the temperature further increases, the liquefied thermosetting resin adheres to the upper and lower substrates 1 and 3, the low melting point particles 7(T) melt, and they wet the conductors 2 and 4 which they are contacting and are metallically bonded to the conductors. At this time, the low melting point particles 7(T) in the spaces 10 between adjoining conductors melt, but they are not contacting the conductors. Therefore, they maintain a spherical shape due to surface tension. If the temperature is further increased and the curing temperature of the thermosetting resin is reached, the thermosetting resin cures and strongly bonds the upper and lower substrates 2 and 4 to each other. When heating by the thermocompression bonding apparatus is ceased, the low melting point particles solidify and completely metallically bond the upper and lower conductors 2 and 4 to each other.
Low melting point particles are superior to high melting point particles with respect to reliability after bonding with an anisotropic conductive material. This is because of the state of bonding between the particles and the conductors. Namely, conduction by high melting point particles is conduction due to the above-described point contact between the high melting point particles and the conductors. In this state, poor contact sometimes takes place. This is because after the upper and lower conductors are bonded with the anisotropic conductive material, if the members which are bonded, i.e., the substrates warp or twist, the strains which they experience are transmitted to the anisotropic conductive material, and the high melting point particles and the conductors which had been in point contact separate from each other. In addition, if electronic equipment which includes a substrate bonded using an anisotropic conductive material also includes devices such as transformers or power transistors which generate heat during use, the interior of a case of the electronic equipment reaches a high temperature during operation, and when operation is stopped, the interior of the case returns to room temperature. When the interior returns to room temperature, condensation takes place everywhere inside the electronic equipment. If moisture due to this condensation penetrates into the anisotropic conductive material, the resin in the anisotropic conductive material swells, and the swelling causes defective contact between the high melting point particles and the conductors in the same manner as the above-described warping or twisting of the substrate.
On the other hand, since low melting point particles are strongly metallically bonded to the conductors, the conductors and the low melting point particles do not readily separate from each other even if a substrate warps or twists or if a resin in the anisotropic conductive material swells due to moisture. This is why an anisotropic conductive material using low melting point particles has superior reliability compared to an anisotropic conductive material using high melting point particles.
Patent Document 1: JP H8-186156 A
Patent Document 2: JP H10-112473 A
Patent Document 3: JP H11-176879 A
Patent Document 4: JP H11-186334 A
Patent Document 5: JP 2002-26070 A
Patent Document 6: JP 2002-217239 A
Patent Document 7: JP 2006-108523 A